Substrate size and heterogeneity control anomalous transport in small streams

Abstract

In alluvial systems, substrate characteristics play a critical role in slowing downstream transport of both water and solutes. We present results from solute injection experiments testing the influence of sediment size (pea gravel versus coarse gravel) and heterogeneity (alternating sections versus well-mixed reaches) on solute transport dynamics in four experimental streams at the Notre Dame Linked Experimental Ecosystem Facility. The stream with pea gravel resulted in more long-term retention than the stream with coarse gravel, whereas both streams with heterogeneous substrate (alternated and mixed) fell between with similar late-time scaling. Inverse modeling of solute breakthrough curves suggested that residence times were distributed according to a truncated power law. While conservative solute transport in all four streams was anomalous, truncation times were influenced by sediment size, with the smaller pea gravel exhibiting a later truncation time than the coarse gravel, and the two streams with heterogeneous substrate having an intermediate cutoff. These results uniquely associate transport scaling with substrate characteristics in fluvial systems, revealing truncation time scales that had been previously predicted but not observed and quantified in field conditions. Because both benthic (i.e., substrate-water interface) and subsurface hyporheic regions are known biogeochemical hot spots, relating physical characteristics to the macroscopic transport behavior could be crucial to improve our estimates of solute export from fluvial systems. Key PointsStreambed sediment controls anomalous transportA truncation of the anomalous behavior is observedOur observations provide a strong foundation for building better models